Microlithography is used for producing microstructured components, such as integrated circuits or LCDs, for example. The microlithography process is carried out in a so-called projection exposure apparatus comprising an illumination device and a projection lens. The image of a mask (reticle) illuminated by the illumination device is projected by the projection lens onto a substrate (for example a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection lens, in order to transfer the mask structure to the light-sensitive coating of the substrate.
Mask inspection apparatus are used for the inspection of reticles for microlithographic projection exposure apparatus.
In projection lenses or inspection lenses designed for the EUV range, i.e. at wavelengths of e.g. approximately 13 nm or approximately 7 nm, owing to the lack of availability of suitable light-transmissive refractive materials, reflective optical elements are used as optical components for the imaging process.
In order to avoid, during operation of a projection exposure apparatus, inter alia a loss of reflection of the reflective optical components as a consequence of contaminants entering the respective optical system, it is known to charge the immediate surroundings of the relevant reflective optical components with an atmosphere of for example hydrogen (as “purge gas”), which prevents ingress of unwanted contaminants into the optical system into the immediate surroundings of these reflective optical components.
In practice, however, the problem arises that this (ionic or atomic) hydrogen penetrates into the multi-layer system (in particular for example a reflection layer system present on the substrate of the reflective optical components and consisting of an alternating sequence of molybdenum (Mo) and silicon (Si) layers), where the hydrogen reacts for example with the silicon, forming volatile silicon hydride (silane), or recombines to molecular hydrogen. These processes lead to an enrichment of gas phases within the reflection layer system and are therefore associated with an increase in volume and “swelling of the layer”, which can ultimately lead to delamination as a consequence of “spalling” of layers of the multi-layer system, and thus to a loss of reflectivity or even destruction of the reflective optical element.
The measurement images shown in FIGS. 6A-6B serve to illustrate the above-described problem of delamination as a consequence of penetrating hydrogen, FIG. 6A showing “bubble formation” in the multi-layer system (referred to as a “blister”) caused by the above-described increase in volume, and FIG. 6B showing delamination resulting therefrom.